Relevant cost

An information technology consultancy firm has been asked to do an urgent job by a client, for which a price of $2500 has been offered. Th? job would require the following:
+ 30 hours’work from 1 member of staff, who is paid on an hourly basic, at a rate of $20 per hour, but who would normally employed on work for client where the charge out rate is $45 per hour. No other member of staff is able to do the member of staff in question’s work.
+ The use of 5 hours of mainframe computer time, which the firm normally charges out to the external users at a rate of $50 per hr. Mainframe computer time is currently used 24hrs a day, 7 days a week.
+ Supplies and incident expenses of $200
Calculate the relevant cost?

The answer in text book:
Labour (30hrs x $20)= 600
Opportunity cost of labour ( 30 hrs x ( $45-$20))= $ 750
Computer opportunity cost (5 hrs x $ 50)= 250
Supplies and expenses 200

But this is my concern:
$600 labour cost is paid to the staff on an hourly basic so where he does this job or not this cost still incurred so it should not be included in relevant cost. the only opportunity cost of labour is $750

please help me, i dont understand this point. Thank you